The present invention relates generally to an air/fuel ratio control system for an internal combustion engine. More particularly, the invention relates to an air/fuel ratio control system for lean-mixture combustion in an internal combustion engine while maintaining engine fluctuations within a predetermined allowable range.
In recent years, lean-mixture combustion has been considered to be good for fuel economy in an internal combustion engine. As less fuel is consumed in each cycle of engine revolution, it is apparent that lean-mixture combustion in the engine will save fuel and provide better fuel economy. On the other hand, lean-mixture combustion has been considered to increase engine roughness and cycle-to-cycle fluctuations in engine revolution. This may degrade engine preformance and drivability.
When the engine running condition is out of the predetermined allowable range, and thus the engine is running in an unstable manner, such unstable conditions may be recognized by checking for variations in the crank shaft angular positions at which the pressure within an engine cylinder is maximized. In general, the crankshaft angular position corresponding to the minimum advance for best torque (MBT) remains constant or at least within a fixed fluctuation range when the engine is running smoothly. On the other hand, when the engine is running unstably or roughly, a variation of the crankshaft angular position at which the internal pressure in the combustion chamber is maximized becomes significant. Therefore, if variation of the crankshaft angular position at which the maximum internal pressure is obtained exceeds a predetermined allowable range, engine roughness or instability can be recognized.
SAE Paper No. 770,217, Feb. 28-Mar. 4, 1977, written by Isao NAGAYAMA, Yasushi ARAKI and Yasuo IIOKA discusses vehicle driveability with reference to FIG. 9 thereof. In the disclosure of this SAE Paper, the driveability limit was set to the point where the driver judged subjectively that the level of vehicle surge produced was unacceptable. The observed relationship between cycle-to-cycle fluctuation of I.M.E.P. and vehicle surge level is shown in FIG. 9 of the SAE Paper. In the test vehicle, especially when it was in third gear, the region of torque fluctuation rate greater than 50% and cycle-to-cycle fluctuation rate greater than 10% exhibitted unacceptable levels of vehicle surge. To aid understanding of the required stability of the engine and, in turn, of roughness of the engine, the disclosure of SAE Paper No. 770217 is hereby incorporated by reference.
As will be appreciated, by making the air/fuel mixture leaner, the cycle-to-cycle fluctuation rate as well as the torque fluctuation rate is increased causing the engine to run roughly. To cure the engine roughness, the air/fuel ratio is controlled to supply a richer mixture. As will be appreciated herefrom, in a lean mixture combustion system, it is essential to detect the engine roughness to perform enrichment in order to prevent the engine from falling into seriously rough operation.